[FTC]: Tips for Teams going to the Vex Championships

As teams prepare for the Vex Championships in a few weeks, the FTA’s from the FTC World Championships want to share some learnings that can make your experience a more enjoyable one. One of the most frustrating things that can happen to a team is losing control of their bot during a match. Before you blame the field, interference, or crystals, consider the following:

Antenna Placement: It is critically important that you keep your antenna wire free of any metal shielding. We found that a high percentage (>20%) of teams laced their antenna wire through the Vex metal. One team even coiled their wire and tied it underneath their bot! Then teams wonder why they lose control during a match. We took one robot that was having problems, unwrapped their antenna from the metal framework, stretched the wire across open space and miraculously all their control problems disappeared.

Excessive Current Draw: Most teams forget that if you draw more than 4 amps there is a likelihood that the controller will reset and restart in autonomous mode. This happens each time you exceed four amps. As robots get more complicated their use of motors is increasing with some bots using the full complement of 10 motors/servos. The problem typically does not reveal itself in the pit, since the drive motors aren’t under the same load as when they are pushing bots on the field. At least one bot with this problem would stop and restart during the match and appeared to be running without operator control.

Damaged Motors: The gears inside the motor can become damaged if they are excessively loaded. We had one robot that had difficulty moving during a match. The signals were strong and there was no obvious interference. When checked later in the pits we found that that three of the four drive motors had stripped gears and the fourth was badly damaged. Once the gears were replaced, the bot ran fine. If your motors are old or have been highly stressed you should consider replacing the gears, after all, that is why a spare set comes with every motor.

Watch the Lights: Make sure the lights on your controller are easily visible. These lights are invaluable when it comes to diagnosing problems on the field. Here is what to look for:
o Power light: should be on during the match, if flashing red then battery is very low
o PGRM: This light should not be on during a match, if it flashes or remains on it indicates problem with code
o “Eye”: Flashing indicates the robot is in autonomous mode.
oRx1/Rx2: Data is being received from the transmitter, if these are not on during a match you have communication problems

Controller Performance: Although the transmitters are pretty rugged it is possible to damage them. At the FTC Championship we used a spectrum analyzer to measure transmitter power and we found 5 or 6 bad transmitters that had intermittent or weak signals. There was even one team that pulled their antenna completely out of the transmitter then tried to push it back in and run in a match. Obviously you are taking your chances when you do this. Not many teams have access to a $50,000 spectrum analyzer to check their transmitter performance but there is a way you can test your transmitter. A good performing transmitter should be able to control a robot 100 feet away. If your system cannot do this, you may want to consider changing the transmitter, replacing the receiver(s), or relocating your antenna.controllers.

Battery Power: No team wants to hear the ref or FTA say that their control issues are caused by low battery, especially when the battery just came off charge, but this is a common cause of problems. Measuring voltage with a voltmeter, or the transmitter voltage monitor, is not a sufficient test for battery health, you really need to test for voltage under a load. Ni-Cads have a problem called voltage depression or lazy battery effect. This is caused when a battery is used for only a short time and then put back on charge for a long period, precisely what most teams do after a match. The battery appears to be fully charged but seems to discharges quickly after a brief period of operation. For a more detailed description go to http://en.wikipedia.org/wiki/Nickel-cadmium_battery If your batteries have been around for a while you may want to consider doing a few deep discharges prior to your next tournament. Using the wrong charging port in the Vex battery charger has also been identified as a problem. You should not use the 9.6 v port to charge the 7.2 v robot batteries. This overcharging can damage the batteries.

Plug in your Rx cables: I have lost count of the number of teams I have had to remind to plug in their Rx cable(s) prior to a match. While the refs and FTA’s try to look for this on the field it is your responsibility to make sure the cables are plugged in correctly. I have also seen many bots with the Rx cable plugged into the serial port. This makes the robot do strange things during a match, very few of which will help you win the match.

**Interference: **During the FTC Championships we continuously monitored the signal traffic using two spectrum analyzers. During that time the only interference we found was a constant signal on crystal 74’s frequency, and an intermittent signal between crystal freqencies 76 and 77. We removed these crystals from use. At no time did we see anything else that would interfere with the robot operation. While this does not guarantee there will be no interference issues during the Vex Championships it should encourage teams to look elsewhere when or if control problems crop up.

Pull the plug: Most teams do not have a clear understanding of how the field operates, they think it sends signals to the robot. In actuality all it does is short a pin in the transmitter preventing it from radiating. The short is removed briefly to start the autonomous mode. Once the remote controlled portion of the match has begun the field controller does nothing. So if you are experiencing trouble controlling your bot the first thing you should do is disconnect the cord connecting your transmitter to the field controller and leave it out. Be aware that the only thing that will stop your bot with the cord disconnected at the end of the match is your Gracious Professionalism.This probably won’t solve your control issues but it will confirm that the field is not the cause of your problem. By the way, when you pull the plug, please squeeze the connector and pull the cord gently, we have had many cords damaged by teams yanking the cord out.

As you can see, there are many possible causes for a robot to lose control or not function as designed. It is not always easy or quick to determine a cause. Unfortunately with the pressure to keep matches on schedule Refs/FTAs do not have the time to diagnose the robot problems. Unless there is clear evidence of a field failure, the Refs will ask the teams to leave and try to solve the problem in the pits.

This is a new one on me. I’ve never heard of it before. We competed at three regional championships and this did not happened to our team before. But it did happen several times at World’s. How can you avoid it?

We ran into this last year when we had a bad controller that would not even deliver 4 amps. We called IFI and they explain the following about the draw of 4 amps. Now we always are careful not to run to many motors under load and we also change out every match with a FULLY charged battery to help the amp drop problem.

This seems a little silly to me. 4 amps is the Maximum that the VEX controller can supply to the motors. I highly doubt that this means that if you draw 4 amps that the controller will shut down. The Controller itself most likely runs off it’s own “line” of power. i.e. those 4 amps are for the motors not the controller.

Sorry to say it is. The battery can only supply so much when discharged and the controller does not know this. Once the motors draw to much and pull the voltage down under something like 6 volts the controller dies and leaves the robot in a loop of rebooting. Thats why charged batteries are key to fixing this problem.

I have seen this to much at competitions. You can tell when this happens by looking at the lights on the side of the controller. If power blinks on and off like some one is turning the power on and off then the controller has crashed. This happens since once the controller crashes it restarts (since the voltage goes back up) but once it turns on it see RC and boots back into auto mode which then crashes the controller again since it tries to draw power again.

It happened to us to in 2nd match of the semi-finals of the 2006/2007 championship season at Atlanta.

20 seconds into teleoperated mode our robot went back into autonomous mode, stripping numerous gears and wasting valuable time against simbiotics.

that was the only time its ever happened to me though.

Precision is important when using English. Let me see if I can untangle the causes and effects being discussed here.

4 Amps is max current the Vex microcontroller will supply through the sum of all of its PWM/Motor ports.

Attempting to draw more than 4 Amps through the PWM/Motor does not cause a Vex microcontroller to reset.

Drawing current out of the Vex batteries will result in a voltage drop at the battery’s output terminals.

If the current drawn is enough to lower the Vex battery’s voltage far enough below 5 Volts, the Vex microcontroller will shutdown and be reset when it starts back up.

Sometimes this happens during matches when the loads the motors are experiencing rise and then fall and when the robot’s battery isn’t up to the task of keeping the robot healthy. The current drawn rises and falls. The battery voltage falls and then rises. The microcontroller shuts down when the voltage drops, and restarts when the voltage rises. At start-up the microcontroller starts executing it’s user code, beginning with the first instruction of that code.

Blake

Now, This all makes sense and I believe it.

And this reminds me, Our team, Twisted Bots, owes a big THANK YOU to Blake and his team for loaning us batteries to get us through the finals. Ours gave up.

Blake,
Could you please post details on the battery charger that discharges the batteries so they work better?

Thanks,
Jon T
Coach Team 177
Twisted Bots

How do we know which is which? The 2 charging slots look identical, and there doesn’t appear to be any labeling. In the documentation for the Vex power pack, Instruction #2 says “Plug the 7.2 V Battery Pack into one of the two white ports on the front of the Battery Charger…” then “Plug the 9.6 V Battery Pack into the other port…”

We never knew there was a difference between the 2 slots.

There isn’t – just make sure the batteries are securely seated in the charger.

Jon,

  1. I didn’t spot your question when you first posted it. This answer is wordy; but hopefully it is also worth reading.

  2. You were very welcome!

  3. For as long as I can remember NiCad batteries being around, I have heard about something called the memory effect. There are other similar names for it. Whatever it is called there is a real observable effect that can bite you in a competition.

The bottom line is that drawing just a little of a NiCad cell’s stored charge out and then recharging that cell, tends to decrease the total amount of current you can draw out of the cell in later uses. Doing that over and over again (take out a little charge and then put a little charge back into the cell). will drastically reduce the amount of charge you can withdraw from the cell.

Using a multi-cell Vex 7.2V Battery in a single competition might hypothetically pull about (2.5/60)Hrs * 4000 mAmps = 167 mAmpHrs out of it (the actual amount drained will typically be significantly less).

The Vex 7.2V batteries are designed to hold 2000 mAmpHrs (mAH) of charge.

If after using a battery in a single match (or after a period of light use during a team meeting) you slap it into a charger and replace those 167 mAH; and if you do this a over and over again, after a while, the battery is not going to be able to supply 2000 mAH. In fact, I predict that it is going to fail at supplying even the full 167 mAH (or significantly less!) that you have been pulling out and putting back in.

The failure will be manifested in a very dramatic drop in the Voltage the battery can sustain across its output terminals. The result of that voltage drop under load is something that I/we covered in earlier messages in this thread.

It is very difficult (well nigh impossible without meticulous measuring and record keeping) to tell the difference between a battery (#1) that sustains 7.8V under no load, but that only has 200-300 mAH available and will suffer a dramatic voltage drop if asked to supply much those mAH quickly to satisfy a high load; and a battery (#2) that shows 7.9V under no load, but will be able to keep its voltage up pretty high while supplying the 167 mAH (max) needed in a match (because the battery is able to supply nearly 2000 mAH if necessary, and its voltage won’t drop badly until the charge is nearly exhausted).

This “memory effect” is, at least partially, reversible.

You reverse it in essentially the same way that you avoid it in the first place; by draining a cell before you recharge it.

Always attempt to completely exhaust a battery before recharging it. It is exhausted when applying a load causes the battery’s voltage to drop quickly (from >7V down toward 5V and lower). In a Vex robot, a sign of this will be seeing the microcontroller’s Red LED (in the eye part of the indicator panel) blink on each time you tell a motor to do some substantial work (like move the bot around).

Another way to exhaust a battery will be to use a commercial charger/discharger. These should have circuitry that prevents you from draining any of the battery’s cells all the way down to 0.0V. According to my charger’s manual, draining them that low can damage the ones that reach 0V first by causing current to flow backwards through them.

Using a big resister or other “dumb” device to drain a battery of cells is another method but it is not recommended for the reason cited in the paragraph immediately above.

Discharging a battery that is in good condition, before recharging it will help keep it in good condition.

A battery that is in bad condition because of often being partially discharged and then recharged; can be brought back to some measure of its original health (capacity) by running it through many “deep” discharge, then recharge, cycles.

I have some batteries that I got from eBay that don’t hold the full 2000mAH that they should. They have improved a bit as I have run them through many discharge/recharge cycles; but they are still far from holding the 2000mAH that they should. They might have permanently lost some of their storage capacity. Others have become dramatically better after running through several cycles.


Related but separate topic.
I bought a SuperBrain 977 charger so that I could keep track of how much charge each of my batteries can hold. The 977 has a clumsy user interface; but that said, it lets me do the following:

  • Charge my NiCad packs using either a typical 120VAC socket as a power source or use a 12VDC car battery.
  • Keep track of how much charge gets stored in a battery as I charge it.
  • Keep track of how much charge I can extract from a battery as I discharge it.
  • Vary the charge/discharge rate (measured in Amps) the charger uses.
  • Vary size of the voltage dip the charger looks for to tell that a battery is fully charged.
  • Charge two batteries at a time using independent settings, or charge one and discharge another.

When I bought my 977 I paid about $130 for at a local hobby shop.


Blake
PS: Some teams (including mine) bad-mouthed their batteries incorrectly - I think that we were suffereing from overheating motors, and the symptoms looked like exhausted batteries - When I checked the batteries on the charger/discharger, they were doing fine - They could supply lots of Amps for far longer than one match without a dramatic voltage drop.

Lol, sorry I never been a Englizh or Spellzings person. I guess I am doomed to an engineer’z fate. :smiley:

And what you said is exactly what I was trying (and failing) to explain.

Practice, practice, practice.